1. Field of the Invention
This invention relates to aircraft gas turbine engine nozzles and in particular to removable baseplates for the divergent seals in the nozzles.
2. Description of Related Art
A gas turbine engine produces a reaction thrust by ejecting a high velocity stream of gas from an exhaust nozzle. Air enters the engine and is compressed in a compressor section, fuel is added to the compressed air and at least a part of it is combusted in a combustor section, and the hot gas is then expanded in a turbine section. Part of the produced energy is extracted to drive the compressor section. Often, particularly in the case of military engines, the turbine effluent is mixed with additional fuel and combusted a second time in an afterburner before being expanded through an exhaust nozzle which converts the remaining available energy of the gas stream into a high velocity flow producing thrust for propulsion power. Thrust developed by the engine can range from a few hundred pounds to many thousand pounds.
A typical aircraft gas turbine engine uses a high pressure converging-diverging exhaust nozzle having alternating convergent nozzle flaps and seals mounted circumferentially around the exhaust section. This convergent section of nozzle is typically capable of adjustably reducing the cross-sectional area through which exhaust gas flows, forming a variable area nozzle throat at its aft end. Mounted immediately aft of the converging nozzle flaps in a similar manner and hinged thereto are corresponding divergent nozzle flaps and seals. One particular application for the present invention is an advanced axisymmetric vectoring nozzle (AVEN.sup..TM. nozzle) wherein the convergent and divergent flaps are connected by a universal joint such as a ball joint to vector the nozzle's exhaust flow and thrust. Further reference may be had to U.S. Pat. No. 4,994,660, entitled "AXISYMMETRICAL VECTORING EXHAUST NOZZLE", by Hauer, assigned to the present assignee.
These divergent flaps increase the exit area through which the exhaust gases flow. At idle and cruise speeds, the nozzle configuration is set to maximize fuel efficiency. However, for take-off and acceleration during flight when maximum thrust is needed, the nozzle is adjusted accordingly to provide the converging-diverging gas flow passage. The dimensions of the throat and exit flow passage areas are varied to match the flow and expansion requirements of varying flight speeds and altitudes experienced on a routine basis by the aircraft.
The flaps and seals are generally rectangular in shape and are about three to six inches wide. The flaps are mounted so that they move about pivot points toward or away from a centerline extending the length of the engine. Each set of flaps spreads out in a fan-like manner during movement. The seals, which are similar in construction to the flaps, typically are positioned between adjacent nozzle flaps. The seals are mounted to move laterally relative to and between the flaps to form a generally continuous interior surface which directs gas flow in a desired manner. All the exhaust flaps are operably connected to various centering and retaining devices on the back sides of the flaps and seals to simultaneously move together in response to actuating devices used to control the nozzle's throat to exit area ratio and, in the case of the AVEN.sup..TM. nozzle, to vector the nozzle's thrust.
One type of conventional nozzle flap or seal comprises an inseparable assembly of welded components. The generally rectangular-shaped bottom portion or front side of the flap or seal faces inwardly and, as such, is directly exposed to the hot exhaust gas of the engine, which may have a temperature of up to about 1200.degree. C. The back side of the flap or seal is also subjected to high temperature, though considerably less, e.g. up to about 400.degree. C. The extreme transient and steady state temperature cycles experienced by the flaps during use induce high thermal stresses in the bottom portion and ultimately cracking results. Exposure of the front side of the seal or flap to such high temperatures produces thermal stresses which warp and structurely degrade the front side, requiring that the nozzle seal or flap be periodically replaced. Replacement of the distressed hardware involves costly down time and replacement parts, since a complete flap or seal must be disconnected from its linkages and attachment points and a replacement installed.
One proposed solution to this problem is disclosed in U.S. Pat. No. 5,000,386, entitled "Exhaust Flaps", by the present inventor and assigned to the present assignee. An exhaust flap for mounting in a gas turbine engine comprises an elongated frame to which is removably mounted a baseplate, comprising a front side and a retainer means. The baseplate is dimensioned to slide into receiving, longitudinally extending channels or slots, attached to the frame, to substantially cover the bottom of the flap and form a solid bottom surface or front side. An axial retainer means attached to the frame securely holds the baseplate in position yet is readily removed to permit replacement of the baseplate by sliding it out of the channels of the frame. The maintenance time and cost involved in replacing a damaged flap is substantially reduced when the damage is limited to the baseplate. The baseplate of the flap is readily replaced with the exhaust flap installed in the engine merely by removal of the retainer means and removal and replacement of the damaged baseplate with a new baseplate. Complete disassembly of the total flap from the engine is avoided.
Another feature and advantage of the flap in U.S. Pat. No. 5,000,386 is that the frame allows the baseplate to float and isolates the thermal growth of the baseplate from the structural restraints of the main flap structure, often referred to as the backbone. However, such a design is not readily adaptable for use with seals. The high temperature exposure of the flap baseplate does not extend to the longitudinally extending edges of the flap baseplate, because it is shielded by the seals;whereas the seal edges are exposed. The transverse expansion between the longitudinally extending edges of the seal baseplate and the curling of the baseplate due to thermal growth would be constrained by the longitudinally extending channels or slots of the design in the prior art.
In accord with a demonstrated need for a seal design which has a thermally isolated or floating baseplate that is easily removed without requiring demounting of the total seal from the engine, there has been developed an exhaust seal assembly for use in aircraft gas turbine engine nozzles which improve upon that previously used. The seal assemblies have increased service life and reduced maintenance time when baseplate replacement is required.